This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) , or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control gas flows (H2 reformate and air/oxygen) to the fuel cell stack not only during start-up and normal system operation, but also during system shutdown. During the shutdown of a fuel cell system that generates hydrogen from liquid fuel, the anode CO emissions increase and can degrade the stack. Accordingly, a primary concern during shutdown is diverting the gas flows of H2 and air/oxygen around or away from the fuel cell stack and disposing of the excess H2. The H2 and air flows being diverted from the stack during shutdown must also be kept separate to avoid creating a combustible mixture in the system. The stack must also be protected from prolonged (e.g., greater than five seconds) pressure differentials which could rupture the thin membranes in the membrane electrode assembly (MEA) separating the anode and cathode gases. It is accordingly important to ensure that the diversion of gas away from the stack occurs properly on shutdown and is reinstated properly on start-up.
In one aspect, the diversion of gas flows around the stack during shutdown is accomplished with anode and cathode bypass valves. In the fuel cell system adapted for use in vehicular applications, the bypass valves comprise relatively slow-moving automotive type bypass valves. The invention solves the potential problems posed due to failure of a bypass valve to close and divert flow around the stack which can degrade the stack. In particular, inoperability of the anode bypass valve on shutdown can degrade the stack with excess CO in the H2 reformate from the fuel processor. Likewise, failure of the anode bypass valve to open upon start up of the fuel cell system can result in cell reversal. Cell reversal occurs when the fuel cell stack is loaded and not enough H2 is supplied to the anode inlet, causing membrane breakthrough and permanent stack degradation. Accordingly, the invention provides method and apparatus to ensure that the diversion of gas away from the anode occurs properly on shutdown, and is reinstated properly on start up. Here, an anode bypass valve and associated valving arrangement is used to ensure proper diversion and reinstatement of flow.
In another aspect, the present invention provides a methodology and valving arrangement for ensuring that the anode bypass valve is closed during a normal shutdown. If it is determined that the anode bypass valve has not properly closed during normal shutdown, the fuel cell system is put into a rapid shutdown mode in which CO-rich H2 reformate is instantaneously vented from the anode inlet. This protects the stack from CO degradation.
According to one aspect of the invention, the pressure at the anode inlet is compared to the pressure Mat the anode outlet. This anode-side pressure drop across the stack decreases rather quickly during a normal shutdown in which the anode bypass valve works properly. When the anode bypass valve closes, by monitoring the xe2x80x9cgapxe2x80x9d or pressure differential between the anode inlet and outlet during the first few seconds of shutdown, it can be determined whether the anode bypass valve has closed properly. A xe2x80x9cclosedxe2x80x9d bypass valve is defined as a valve position directing all flow around the stack. If the gap between anode inlet and outlet pressures drops quickly to near zero in the first few seconds, the anode bypass valve has closed properly. If the gap between anode inlet and outlet pressures drops slowly, or increases during shutdown, a signal is generated by the fuel system controller or software indicating anode bypass failure and triggering a rapid shutdown. In the rapid shutdown mode, the anode inlet is instantaneously vented by a fast-acting vent in the flow path from the anode bypass valve to the anode inlet.
In another aspect of the invention, pressure sensors are added to the anode inlet and outlet, and optionally any limit switches, wiring, and input/output structure associated with the bypass valving for physical verification of proper operation are removed. The difference in pressures as determined by the sensors at the anode inlet and outlet is carefully monitored at least during a normal shutdown procedure, and the difference tracked over a period of time corresponding to the time in which pressure at the anode inlet can typically be expected to equalize with the pressure at the anode outlet if the anode bypass valve closes properly. If the pressure differential does not significantly decrease over the prescribed period of time, a signal is generated indicating anode bypass failure and the system is switched to a rapid shutdown mode in which the anode inlet is instantaneously vented.
The vent valving for carrying out the invention methodology may comprise existing valves and a fuel cell system controlled according to the invention methodology during a shutdown, or may comprise single-purpose valving added to an existing fuel cell system. Monitoring of the pressure differential via the pressure sensors can be through a dedicated controller comprising any suitable microprocessor, microcontroller, personal computer, etc. which has a central processing unit capable of executing a control program and data storage in the memory. The controller may additionally comprise an existing controller in a fuel cell system. Control of the fast-acting vent valving in rapid shutdown is achieved in similar fashion.